Prevention of corrosion using heterocyclic nitrogen compounds

ABSTRACT

CORROSION OF METALS BY AQUEOUS ACIDIC SOLUTIONS IS MARKEDLY INHIBITED BY THE PRESENCE OF A HETEROCYCLIC NITROGEN COMPOUND HAVING A 5-MEMBER RING CONTAINING 2 OR 3 NITROGEN ATOMS. TYPICAL COMPOUNDS ARE PYRAZOLE AND 1,2, 4-TRIAZOLE. CORROSION OF CHEMICAL AND PETROLEUM PROCESS EQUIPMENT HANDLING HYDROCARBON AND OTHER CHEMICAL STREAMS CONTAINING ACIDIC SUBSTANCES IS MINIMIZED BY THE PRESENCE OF A CORROSION INHIBITOR OF THIS INVENTION.

United States Patent 0 3,553,101 PREVENTION OF CORROSION USING HETERO-CYCLIC NITROGEN COMPOUNDS Zisis Andrew Foroulis, East Orange, N.J.,assignor to Esso Research and Engineering Company, a corporation ofDelaware No Drawing. Filed May 17, 1968, Ser. No. 729,925 Int. Cl. B01d3/24; C23f 11/04, 14/02 US. Cl. 208-47 16 Claims ABSTRACT OF THEDISCLOSURE BACKGROUND OF THE INVENTION This invention relates to theprevention of corrosion of metals by aqueous acidic solutions, and moreparticularly to the prevention of corrosion of chemical and petroleumprocess equipment which is subjected to corrosive attack by aqueousacidic solutions as a result of condensation of water containingdissolved acidic substances.

Various acidic substances which are present in petroleum refiningoperations cause corrosion of metals with which they come in contact.Examples of destructive inorganic compounds include hydrochloric acid,sulfuric acid, sulfur trioxide, and hydrogen sulfide. Organic compoundscausing corrosion include acetic acid, phenolic compounds, naphthenicacids, and aliphatic and naphthenic organic chlorides. Corrosion-causingacids enter the hydrocarbon process streams in pertoleum refineries invarious ways. For example, crude oils generally contain naphthenicacids. The organic chlorides do not usually occur naturally in crudeoil, but are sometimes added by producers for removal of paraffindeposits in producing wells and pipelines. These tend to hydrolyze inthe presence of water to produce hydrochloric acid. Hydrogen sulfide isformed in catalytic desulfurization processes using hydrogen, in whichvarious hydrocarbon feedstocks including virgin and cracked naphthas, aswell as gas oils, containing such impurities as mercaptans, disulfides,and thiophenes, are catalytically reacted with hydrogen in order toreduce their sulfur content. Sulfuric acid and sulfur dioxide are bothprocessing reagents, the former being used as an alkylation catalyst andthe latter as an extractant for the removal of aromatics fromhydrocarbon feedstocks. Hydrochloric acid and hydrogen chloride mayresult from several sources, including the hydrolysis of organicchlorides, hydrolysis of salt which is mixed with crude oil as a resultof the use of brine of oil production operations, and as a result ofhydrolysis of chlorine gas which is used in the regeneration of platinumreforming catalysts.

Corrosion caused by acids is also a problem in the chemical industry.For example, concentrated aqueous phosphoric acid, which is widely usedin the manufacture of fertilizers as a source of phosphorus, isextremely corrosive to carbon steels. Consequently, expensive alloysteels are widely used in process equipment and in storage andtransportation equipment for handling phosphoric acid.

Acidic substances such as the foregoing will cause severe corrosion ofthe metals from which conventional petroleum refining and chemicalprocess equipment'is constructed. Carbon steels, such as 1020 carbonsteel con- 3,553,101 Patented Jan. 5, 1971 taining 0.2% carbon, are usedpredominantly as materials of construction. While it would be possibleto fabricate refinery equipment from steels which are less prone tocorrosive attack, such as stainless steel and special alloy steels, thecost of such equipment would be inordinately high and would make anyprocess being conducted with such equipment uneconomical.

Corrosion in petroleum process streams is particularly troublesome inequipment, such as condensers and heat exchangers, where condensation ofwater takes place. Water vapor is invariably present in hydrocarbonprocess streams, as is well known. When this water condenses, acidicgases present in the process stream, such as hydrogen chloride, hydrogensulfide, sulfur dioxide, and carbon dioxide, dissolve in the condensateand attack metal equipment. Such attack occurs in hydrocarbon processstreams containing only trace amounts of oxygen or none at all, sincethe metal is oxidized by the hydrogen ions of the acid.

The overhead stream from an atmospheric pipestill is one example of apetroleum process stream containing acidic gases. Such a streamgenerally contains hydrogen chloride as well as organic chlorides. Uponcondensation of this overhead stream, hydrogen chloride is dissolved inthe water condensate and the quantity of hydrogen chloride is ugmentedby hydrolysis of a portion of the organic chlorides present. Thiscondensate attacks the condenser surfaces.

One possible technique for inhibiting corrosion of refinery equipment byacids is to neutralize the acid with a base. However, such a solutionwould not be practical because there is a tremendous daily throughput offeed streams through petroleum or chemical processes which containacidic materials, thereby requiring a correspondingly large amount ofbase for neutralization. A further problem arises from the fact that themost likely base for use in such neutralization would be ammonia.However, the neutralization of acidic components such as hydrogenchloride by injection of ammonia in hydrocarbon streams, quitefreqeuntly results in extensive fouling of process equipment, such asheat exchangers, towers, etc., due to decomposition of ammonium saltproduced by neutralization. In addition, in instances such as thephosphoric acid storage facilities, the use of neutralizers clearly iscon traindicated. It is thus evident that a meaningful answer to theproblem facing the petroleum and chemical industries would not be basedon neutralization or removal of the acidic corrosive agents in the feedstream since such techniques would either be prohibitively expensive orwould result in extensive fouling of process equipment. Instead, it isnecessary to provide a corrosion inhibitor whose elfectiveness does notdepend on neutralization of acid present and which does not contributeto fouling.

The use of pyrrole as a corrosion inhibitor for metals exposed to acidicmedia has been previously suggested. While pyrrole does inhibitcorrosion of metals by acids, it tends to polymerize at hightemperatures such as those encountered in petroleum processing, and thusproduces fouling of process equipment.

SUMMARY OF THE INVENTION It has been found that corrosion of metals byaqueous acids can be markedly inhibited by the presence of aheterocyclic nitrogen compound having a S-member ring containing 2 or 3nitrogen atoms. Typical compounds include pyrazole, imidazole, and1,2,4-triazole.

DETAILED DESCRIPTION The corrosion inhibitors of this invention arearomatic compounds which have a S-member heterocyclic ring containing 2or 3 nitrogen atoms. Among the effective corrosion inhibiting compoundsare pyrazole, imidazole,

1,2,3-triazole, 1,2,4-triazole, and derivatives thereof. The parentcompounds may be represented by the following structural formulas:

pyrazolc iniidazole l N n l N H N 1, 2, 3-triazole 1, 2, 4-triazole isan excellent corrosion inhibitor. The derivative may be a monocycliccompound in which one or more hydrogen atoms of the parent are replacedby substituents, or may be a compound having one or more rings, usuallyaromatic or alicyclic rings, fused to the heterocyclic ring. Forexample, benzimidazole, having the formula is a corrosion inhibitor.

To be good corrosion inhibitors, compounds are preferably moderatelysoluble in water. The water solubility of the compound must besufficient to provide an effective corrosion inhibiting concentration.Hence, derivatives having long-chain alkyl substituents, and polycycliccompounds having more than two rings, generally are not effectivecorrosion inhibitors. Desired water solubility can be achieved incompounds having both solubilizing groups, i.e., hydroxyl, sulfonatesalt substituents, and insolubilizing groups such as long-chain alkylgroups.

The inhibitors of this invention may be used effectively in widelyvarying concentrations. Effective inhibition is obtained inconcentrations as low as about moles per liter up to about 0.5 mole perliter of the aqueous acidic phase. Actually, there is no upper limit onthe effectiveness of the inhibitors of this invention, and the maximumconcentration is limited only by the solubility of the compound.However, concentrations in excess of 0.5 m./l. do not give inhibitoryaction substantially greater than that obtained at concentrations under0.5 m./l.

The nitrogen heterocycles of this invention function most effectively inhydrochloric, sulfuric and phosphoric acids. These compounds offereffective protection against corrosion by non-oxidizing acids as well asagainst corrosion by oxidizing acids.

Any metals which are subject to acid attack can be protected with theinhibitors of this invention. These inhibitors are particularly usefulfor protection of ferrous metals, and especially low carbon steel, suchas 1020 carbon steel (containing 0.2% carbon). Low carbon steels areideal for construction of petroleum processing equipment from thestandpoint of cost and other significant qualities such as strength andtheir ability to withstand the process stream temperatures. Theprincipal drawback to low carbon steel is its susceptibility to acidcorrosion, and problems arising from this are substantially obviated bythe use of the inhibitors of this invention.

Corrosion by acids is ordinarily a problem where the pH of the acidicsolution is about 4 or lower. The in- 4 hibitors of this invention offerexcellent protection even in solutions which are decidedly on the acidside, e.g., those having a pH of 1 or lower.

A few types of apparatus used in the petroleum and chemicals processingindustry will be cited as examples of apparatus, which as indicatedearlier, the presence of aromatic heterocyclic compounds serves toinhibit the corrosion effects of various acid base corrosion causingsubstances. An area where corrosion is quite serious is the overheadequipment, i.e., condensers and other hardware, in atmosphericdistillation towers. Substantial quantities of water vapor and hydrogenchloride distill overhead together with low molecular weighthydrocarbons during atmospheric distillation of crude oil. The hydrogenchloride present in the overhead stream dissolves in the watercondensate and attacks the overhead condensers and other relatedequipment. Corrosion is markedly reduced by injection of a nitrogenheterocyclic compound in the process stream.

Another area where corrosion is widespread and has a deleterious effectis hydrotreating of petroleum fractions. Substantial quantities ofhydrogen sulfide are produced in the hydrotreater by reduction of sulfurcompounds such as mercaptans, disulfides, and thiophene. This causessevere corrosion, particularly in the presence of water condensate. Alsopresent is hydrogen chloride, which results from the decomposition oforganic chlorides such as carbon tetrachloride and trichloroethane inthe process stream. In any event, the hydrotreater effluent condenserand other overhead equipment has been plagued with problems instigatedby the presence of hydrogen chloride. Again, corrosion is greatlyreduced by the injection of a nitrogen heterocycle into the processstream.

Another area where the inhibitors of this invention can be usedeffectively is to control corrosion in phosphoric acid storagefacilities and other related process equipment where phosphoric acid isused in fertilizer manufacture. The nitrogen heterocyclic compound maybe directly injected in the process stream.

A significant advantage of the use of corrosion inhibitors is that it ispossible to use inexpensive construction materials such as low carbonsteel, instead of costly corrosion resistant alloy steels which wouldrender the cost of the process prohibitive.

While ferrous metals have been cited as an ilustrative example of metalswhich can be protected according to this invention, it should beunderstood that other metals and alloys, such as nickel, zinc, brass,and copper, may also be protected. While copper is more resistant toacid attack in a non-oxidizing atmosphere than the other metals andalloys mentioned, nevertheless it may be prone to slight attack bystrong acids, and such attack is mitigated by aromatic heterocyclicnitrogen compounds.

The problem of corrosion attack is most severe in those units, such ascondensers, heat exchangers, and transfer lines, where water condenses.The acid gases present in the process stream are dissolved in thecondensate, and attack the metal process equipment. It has been foundthat the corrosion inhibitors herein are effec tive under the entiretemperature range in which water is present in the liquid phase. Sincesome processes are run at high pressure, the actual temperature may beconsiderably above the atmospheric boiling point of water; nevertheless,the inhibitors do not lose their effectiveness at such temperatures.Likewise, they remain effective at low temperatures down to 32 F.

The inhibitor is preferably injected into the process stream just ashort distance upstream for best results. This mitigates loss of theinhibitor, and also protects the inhibitor from decomposition from hightemperatures which prevail in some units of process streams.

While the mechanism for the inhibiting action of aromatic heterocyclicnitrogen compounds is not completely understood, the followingexplanation is offered for the purpose of illustration and as an aid inunderstanding the invention, and should not be taken as limiting thescope of the invention in any manner. The corrosion additive is believedto be adsorbed on the metal surface in the form of a continous or nearlycontinuous thin film. This film would serve to inhibit any chemical orelectrochemical interaction between the acidic corrosive material insolution and the metal surface. The very small quantities of inhibitorthat are utilized to form this thin film are not believed to undergo anysignificant chemical reaction with the acidic corrosive material. Thus,if at all, only small amounts of additional inhibitor would be necessaryto maintain long term protection on metal surfaces, these additionsbeing possibly necessitated by attrition losses due to physicalinteractions of the flowing stream with the film.

As previously noted, the corrosion inhibitor should not be markedlywater soluble, nor should it be substantially completely insoluble. Inshort, the water solubility must be enough to establish an effectiveconcentration, which as earlier noted is generally at least 10- m./l.

The present invention will be more fully understood with reference tothe following specific examples. It is understood that these examplesare illustrations of specific embodiments of this invention and are notto be taken as limitations.

EXAMPLE 1 This example illustrates the efiicacy of 1,2,4-triazole as aninhibitor of acid induced corrosion of 1020 carbon steel exposed to 0.1N hydrochloric acid, which has a pH of 1. Corrosion rates were measuredby weight loss of carbon steel specimens having a size of approximately2.5 cm. x 0.6 cm. x 0.3 cm., and a surface area of approximately 15square centimeters. The specimens were abraded through 4-0 emery paper,degreased in benzene, and washed in distilled water. Immediately afterdrying, the specimens were weighed and placed in a corrosion cell animmersed in the corrosive solution. Each of the corrosive solutions,except those used for control purposes, contained a predeterminedconcentration of 1,2,4-triazole. The amount of corroded metal wasdetermined by weight loss, The corrosion cell was basically a 2000 ml.Erlenmeyer flask with a special top to permit entrance and exit fornitrogen for deaeration and to prevent air contamination. The cell had aremovable chimney with Pyrex hooks from which the metal surfaces weresuspended. The corrosion solution was deaerated with nitrogen before arun. Nitrogen was also bubbled through the solution continuously duringa run to prevent contamination with air. A constant temperature wasachieved by the use of constant temperature oil bath. All runs werecarried out for two days at a constant temperature of 25 C.

The results of representative experiments utilizing the above procedureare summarized below in Table I. In this table, corrosion rate inmilligrams per square decimeter per day (mg./dr. /day or mdd.) andpercentage inhibitor efficiency (which equals where I is the corrosionrate without inhibitor arid Ii is the corrosion rate with inhibitor) aregiven for various concentrations of inhibitor in moles per liter(m./l.).

EXAMPLE 2 The procedure of Example 1 was followed except that theinhibitor was 3-amino-l,2,4-triazole. Results are summarized in TableII.

The procedure of Example 1 was repeated except that a the inhibitor waspyrazole. Results are summarized in The procedure of Example 1 wasrepeated except that the inhibotor was 3,5-dimethylpyrazole. Results'aresummarized in Table IV.

TABLE IV Inhibitor Corrosion Percent concentration, rate (mdd.),inhibitor m./l. mgJdmJ [day elficiency The corrosion inhibitorsdescribed herein give substantial protection against corrosion of metalby acids. These inhibitors are thermally stable, retaining theireffectiveness at the elevated temperatures encountered in hydrocarbonprocess streams.

While this invention has been particularly described with reference topetroleum processing streams, it will be understood that similar acidcorrosion problems may also occur in chemical processing equipment andin containers in storage and shipment of acidic materials. Suchequipment and containers can also be protected with the corrosioninhibitors of this invention.

What is claimed is:

1. A process for preventing corrosion of a ferrous metal by an aqueoushydrochloric acid solution having a pH not greater than about 4 whichcomprises adding to said solution a corrosion inhibiting amount of aheterocyclic nitrogen compound having a five-member ring containing 2 or3 N atoms.

2. A process according to claim 1, in which said compound is1,2,4-triazole.

3. A process according to claim 1, in which said com-- pound is3-amino-1,2,4-triazole.

4. A process according to claim 1, in which said compound is pyrazole.

5. A process according to claim 1, in which the concentration of saidcompound is in the range of about 10- m./l. to about 0.5 m./l.

6. A process according to claim 1, in which said solution and thesurrounding atmosphere are non-oxidizing.

7. A process according to claim 1, in which said acidic solution is acondensate in a hydrocarbon process stream.

8. A process according to claim 1, in which said corrosion inhibitingcompound is added to a hydrocarbon process stream upstream of the areato be protected.

9. A process for inhibiting corrosion caused by an aqueous hydrochloricacid condensate having a pH less than about 4 in a ferrous metal vesselcontaining a hydrocarbonaceous fiuid, said process comprising adding tosaid vessel a heterocyclic nitrogen compound having a five-member ringcontaining 2 to 3 N atoms.

10. A process according to claim 9, in which said vessel carries ahydrocarbon process stream.

11. An aqueous hydrochloric acid solution having a pH less than about 4and inhibited against corrosive attack on metals, said solutioncomprising water, an acidic substance normally tending to causecorrosion of metals, and a small but eifective corrosion inhibitingamount of a heterocyclic nitrogen compound having a five-member ringcontaining 2 or 3 N atoms.

12. A solution according to claim 11, having a pH not greater than about4.

13. A solution according to claim 11, in which said compound is presentin a concentration of about 10 m./l. to about 0.5 m./l.

14. A solution according to claim 11, in which said compound is1,2,4-triazole.

=15. A solution according to claim .11, in which said compound is3-amino-1,2,4-triazole.

16. A solution according to claim 11, in which said compound ispyrazole.

References Cited UNITED STATES PATENTS 2,908,640 10/1959 Dougherty 2039X3,222,285 12/1965 Rai et al 252394X 3,382,087 5/1968 Ostrowski 252-3903,408,307 10/1968 Troscinski et a1 252-394 3,414,519 12/1968 Beynon252390X 3,452,038 6/ 1969 Randall et a1 252390X DELBERT E. GANTZ,Primary Examiner G. E. SCHMITKONS, Assistant Examiner US. Cl. X.R.

